Save Space and Speed Up Your Web pages by Converting Gifs to Jpgs

By Joe St Sauver (joe@oregon.uoregon.edu)

Many web designers are used to relying primarily on gif-format images, when often jpg-format images are actually preferable.

For example, jpgs tend to be smaller than comparable gifs for continuous-tone photographic images, and they also render color more faithfully.

If you have thousands of existing images in gif format, the thought of converting them all to a different format might be a little daunting. Fortunately, however, you don't need to convert them one at a time. Instead, you can use this simple script on DARKWING to convert them en masse:

#!/bin/csh -x

foreach f ( *.gif )

set base = `basename $f .gif`

giftopnm $f | cjpeg -quality 50 > $base.jpg

end

(Note that the backticks on line three are just that: backtick marks, not apostrophes.)

The script assumes that your gifs have the suffix .gif and reside in your current directory. It also shows a typical JPEG quality factor of 50; higher or lower values may be appropriate if you want better image quality or better image compression. For seat-of-the-pants calibration, a sample image I converted had the following quality versus size relationship:

Original gif: 220520 bytes

QUALITY	SIZE
jpg @ quality 90	149242 bytes
jpg @ quality 80	97294 bytes
jpg @ quality 70	71116 bytes
jpg @ quality 60	56573 bytes
jpg @ quality 50	48203 bytes

A Few Caveats

• The resulting jpgs are left in the same directory.
• You'll need to adjust any web pages referring to the old gif images so that they point to the new jpg images instead.
• Be sure to check the protections on the new jpg files to ensure that they're properly accessible.
• This conversion is intended for use with generic gifs that need to be converted into generic jpgs; if you're using gifs which exploit extensions, this script probably won't work for you.

See man giftopnm and man cjpeg for more information on those two commands. If you run into problems or have questions, feel free to send mail to joe@oregon.uoregon.edu